This invention relates to a method of and apparatus for fabricating an optical glass base material by means of a VAD method (vapor-phase axial deposition method).
The VAD method has been already used as means for fabricating glass base materials of products such as communication optical fibers, image guides, light guides, rod lenses and the like.
In the VAD know method, vapor-phase glass stock, oxygen gas, hydrogen gas, buffer gas (inert gas) and the like are supplied to a reaction burner of a multi-wall tube structure, the chemically reacted product produced through the burner in the combustion state, i.e., spot-like fine vitreous particles are injected to and deposited on a target, and porous rod-shaped base material is thus formed.
Then, the rod-shaped base material is transparently vitrified to form a preformed rod.
In the general VAD method, a target is lifted synchronously with the growing velocity of the rod-shaped base material which grows in a vertical direction, and a so-called "a vertical type" is employed, but in the case of this type, when an apparatus for depositing and forming the rod-shaped base material, an apparatus for transparently vitrifying the rod-shaped base material and further an apparatus for cutting the base material thus transparently vitrified are used so that, the entire height reaches approximately 10 m. When the optical glass base material is fabricated by such conventional apparatus, the following problems and drawback exist:
(i) Since the porous rod-shaped base material is transparently vitrified, the viscosity of the part which is lowered by the heat treatment at that time and the part cannot support the weight of the porous portion continued underneath the part, the length and the thickness of the rod-shaped base material formed by the above deposition has to be reduced, and the long and thick base material cannot be produced. PA1 (ii) When the rod-shaped base material is thus transparently vitrified as described above, the porous rod-shaped base material is abruptly and largely shrunk in volume to decrease in the diameter and length to approximately 1/2 (approximately 1/2 in volume). In this case, this shrinkage of the base material occurs irregularly due to the irregular density of the porous rod-shaped base material and to the irregular temperature distribution at the transparently vitrifying time, and bending occurs between the transparently vitrified part and the porous part of the base material. PA1 (iii) Since the entire apparatus has a great height such as 10 m, its driving operations are difficult, and it is necessary to provide a remedy against the safety factor of the apparatus in view of the structural strength, and its housing should accordingly be large in size which is an economical disadvantage. PA1 (iv) When the rod-shaped base material is deposited and grown in the vertical direction, a rising gas stream due to the flame of the reaction burner flows to the porous portion of the upper part which is already formed, and has an adverse effect in the outer diameter and the distribution of the surface components of the porous portion.
When such a bending takes place in the base material, the grown end (lower end) of the rod-shaped base material is displaced from the center of the deposited position, and a fluctuation occurs in the base material of rotating state. Accordingly, the fabricating conditions for obtaining a high quality base material are disorganized.